The invention relates to making labyrinth seal lips for the moving (rotary) parts of turbomachines, in particular for the rotors of gas turbine engines for aircraft.
In such gas turbines, the clearance between the radial ends of the moving parts and the facing stationary parts must be as small as possible in order to avoid lowering efficiency. This applies both between the tips and of the moving blades and a stationary ring surrounding them, and also between the periphery of a rotor and the tips of the stationary vanes situated around the rotor. In order to prevent gas flowing directly between the tips of the stationary vanes and the rotor, it is also known to form projecting annular portions on the periphery of the rotor that are referred to as xe2x80x9clipsxe2x80x9d, at least some of which can define a tortuous path or labyrinth.
The small size of the clearance between the stationary parts and the moving parts, and also the variations in dimensions due to temperature changes, mean that the radial end portions of the moving parts can come into contact with the fixed parts. In order to avoid such contact or friction being damaging, it is known to coat the fixed parts with a so-called xe2x80x9cabradablexe2x80x9d material that is softer than the material of the radial end potions of the moving parts. Thus, if contact is made, then the end portions of the moving parts penetrate into the abradable material, wearing it away, thereby enabling some minimum clearance to be maintained between the fixed and moving parts. Abradable materials are based on metals, for example they can be based on nickel, cobalt, or iron, said metals being compatible with the material from which the fixed parts are made. Abradability can be obtained by conferring a certain amount of porosity to such materials.
Titanium is a material that is being used increasingly for making moving parts, in particular rotor parts in gas turbine engines for aircraft. High energy contacts between materials containing titanium and materials containing nickel, for example, lead to fragile eutectics being formed having low melting points and presenting a high degree of shrinkage on solidifying, which can lead to cracks being started. On propagating, such cracks can become transformed into fatigue cracks in the metal that can in turn lead to destruction of the moving parts.
That is why it is known to provide the end portions of titanium or titanium alloy moving parts with a protective coating to prevent direct contact being made between titanium and the abradable material of the stationary parts. Such a protective coating is generally of ceramic type, being abrasive in character, and is typically made of alumina. The coating is deposited on the end portions of the moving parts, in particular on the labyrinth seal lips of rotors, by thermal sputtering using a carrier gas at high speed, with the thickness of the coating generally being restricted to 0.1 millimeters (mm) or 0.2 mm.
For rotors that are made up of sections that are welded together like the rotor 1 in FIG. 1, the lips 2 and 2xe2x80x2, and in particular the labyrinth seal lips 2, must be coated after the rotor has been assembled in order to prevent the coating being damaged during welding. Unfortunately, the shape of the labyrinth seal lips 2 and their offset location between the moving blades 3 at a distance from the welding connection plane 4, makes it very difficult or even practically impossible to achieve a completely satisfactory coating on both flanks of the lips, i.e. a coating that guarantees there will be no contact between the titanium of the material constituting the rotor 1 and the abradable material 5 fitted to the tips of the stationary vanes 6 facing the lips 2.
An object of the invention is to provide a method of making labyrinth seal lips at the periphery of a metal moving part of a turbomachine, and particularly but not exclusively a part made essentially of titanium or titanium alloy, which method avoids the above-mentioned drawbacks of the prior art, in particular by making it possible to avoid any risk of contact and consequent eutectic formation between the metal material of the moving part and any abradable material carried by an adjacent stationary part, and to make this possible regardless of the shape and the location of the lips.
This object is achieved by the fact that prior to assembly of the moving part, a thick layer of refractory material that adheres to the metal is formed on the moving part, said refractory material containing at least a metal or a ceramic, and labyrinth seal lips are machined to their final dimensions in the thick layer of refractory material.
The refractory material is selected in such a manner as to be compatible with the metal of the moving part, i.e. to be capable of adhering to the surface of the metal and having a coefficient of thermal expansion that is close to that of the metal.
Naturally, the refractory material is also selected so as to avoid damaging eutectics being created by high energy contact with an abradable material. The refractory material therefore preferably does not include any titanium.
Particularly, but not exclusively, when the metal of the moving part is essentially titanium or titanium alloy, the refractory material preferably comprises a metal such as iron, nickel, or cobalt, together with a ceramic selected for example from borides, nitrides, carbides, and refractory oxides, and in particular at least one ceramic of abrasive nature, typically alumina, silicon or tungsten carbides, or boron nitrides.
The layer of refractory material is of a thickness selected to enable the labyrinth seal lips of the moving parts to be machined in said thickness. Prior to machining, the thickness of the layer that is formed is greater than 1 mm, typically lying in the range 3 mm to 8 mm.
The layer of refractory material is advantageously in the form of agglomerated powder or powder mixture. It can be formed by thermal sputtering, in particular by plasma deposition, or by brazing onto the metal.
The invention also provides a moving part forming a turbomachine rotor having labyrinth seal lips of the kind that can be obtained by the above-defined method, i.e. lips machined in a thick layer of refractory material.